Well surveying method and apparatus



Je 13, 1944. E FEARON 2.351.028

WELL SURVEYING METHOD AND APPARATUS Filed Sept. 17, 1941 3 Sheets-Sheet1 I v lllllllllll u u l June 13, 1944. R FEARON 2,351,028

WELL SURVEYING METHOD AND APPARATUS Filed Sept. 17, 1941 3 Sheets-Sheet2 June 13, R FEARQN WELL SURVEYING METHOD AND APPARATUS Filed Sept. 17,1941 3 Sheets-Sheet 5 Patented June 13, 1944 WELL SURVEYING METHOD ANDAPPARATUS Robert Earl Fearon. Tulsa, Okla... asslgnor to Well Burn".Incorporated, Tulsa,

tion of Delaware th., a corpora- Application September 1'1, 1941, SerialNo. 411,228 6 claims. (Cl. 250-835) This invention relates togeophysical prospecting and more particularly to a method and apparatusespecially adapted to determine the nature of strata adjacent a borehole by a spectral analysis of the gamma rays emerging therefrom intothe bore hole.

Prior to this invention investigations of geological formations bothfrom the interior of bore holes and from the surface of the earth havebeen made by detecting and measuring gamma radiations emerging from theformations. In every instance prior to the present invention, however,except for those described in a patent granted October 22, 1940, toSerge A. Scherbat-s- Key No. 2,219,274, and an application of JacobNeufeld filed March 9, 1940, and assigned the Serial Number 323,239,which matured as Patent Number 2,296,176, the investigators have beencontent to measure the total intensity of the gamma rays emanating fromthe various formations and have not attempted to analyze theseradiations to obtain further information.

In the investigation of Neufeld, Scherbatskoy and the present inventor,however, it has been discovered that an analysis of the nature of thegamma rays emanating from various formations often yields importantadditonal information as to the nature of the formation. The gamma raysfrom thorium C" have a wave length of around 4.66X IO-cm. whereas thosefor radium B+C have a wave length of around 1323x10- cm. Other materialsemit gamma rays of still other wave lengths and hence the gamma raysfrom various formations usually differ in their wave lengths dependingupon the material in the formation that gives rise to them. Thus, if notonly the total intensity but also the spectral distribution of the gammarays can be ascertained a further basis is obtained for distinguishingbetween various formations.

In using the Geiger-Muller counter for the measurement of gamma rays ithas been customaryto apply a potential to the electrodes sufllcient sothat the ionization caused by the action of a single quantum of gammarays upon a molecule of the gas will be sufficient to cause anelectrical break-down that will permit a current to flow untilinterrupted by the action of the ballast resistance in the externalcircuit of the counter tube. The current that flows has no relation inmagnitude to the frequency of the gamma rays and it is only by countingthe number of break-downs that occur during a given period of time thata determination is made of a substantially fixed proportion of the totalnumber of gamma ray quanta at the point of measurement.

Recently there has come into more general use in place of theGeiger-Muller counter tube, an ionization chamber in which a higherpressure of gas is maintained. With this higher pressure of gas,ionization occurs practically continuous]! between the electrodes butthe potential applied is not sufficient to cause the break-downphenomenon to occur and hence there results in the external circuit ofthe ionization chamber only a continuous current flow of varyingmagnitude instead of a series of electrical impulses. The magnitude ofthis continuous current is an indication of the total amount ofionization produced by the gamma rays and hence of the total energyexerted by the gamma rays in causins the ionization. Again, however,nothing is revealed as to the frequency of the gamm r y In the patent toScherbatskoy and in the application of Neufeld, mentioned above,measurements of gamma rays are made with two ionization chambers one ofwhich is shielded to a greater extent than the other so as to eliminateat least a portion of the lower frequency, less penetrating, gamma raysfrom measurement by that ionization chamber. By a comparison of themeasurements made by the two ionization chambers an indication of theaverage wave length of the gamma radiation is obtained. This indication,however, still fails to indicate whether all of the gamma rays are ofsome particular intermediate frequency or whether the average is made upof gamma rays of low frequencies and gamma rays of high frequencies.

According to this invention, it has been noted that the energy includedin a quantum of gamma rays is proportional to the frequency of the gammarays in accordance with the formula E=lw where E is the energy perquantum, h is Plancks constant and v is the frequency. Thus, if theentire energy of a quantum of gamma rays is absorbed in the productionof ion pairs and the total production of ion pairs is measured, themagnitude of the measurement will increase as the frequency of the gammarays increase. Under these circumstances, if each quantum of gamma raysis separately detected and measured by the ionization chamber circuitwithout any break-down phenomenon occurring which tends to make allmeasurements of equal intensity, but instead with the natural intensityof each measurement being recorded, then a record will be produced ofnot only the total number of gamma ray quanta detected but also of thefrequency of the gamma rays constituting each quantum.

The present invention overcomes these diiliculties and provides a methodand apparatus for rapidly and efficiently detecting and measuring thewave lengths of a representative sample of the gamma rays at any givenlocation. Tothis end an ionization chamber has been designed which hassufllcient volume in relation to its gas pressure so that when a quantumof gamma rays enters the chamber and reacts with the molecule of the gasto liberate one or more beta particles,

the beta partcleg will in the great majority of instances havelulllcient opportunity to completetheirtravelandrormasmanyionpairs asthey are capable of iormlns. Electrodes are placed in this imisationchamber in close enough Proximity to each other so that the ions formedare efilciently collected and the potential applied to theelcctrndesissuch as toiacilitatetluscollection without causing anybreak-down phenomenon to occur. With all or this, thechamberisconstructedtobeotsuchsiaeandtocontain such a concentration ofgas as will cause the absorption of a sumcient number of quanta of gammarays to make pdssible rapid and accurate measurement without at the sametime causinganabsorptlonoisomanygammarays that their characteristicscannot be separately recorded.

In order to record the characteristics of the gamma rays so detectedwith suilicient speed so that there will not be appreciably overlappingbetween the recordings or two or more quanta of gamma rays it has beennecessary to employ a recording circuit tor the detector that has a verylow time lag. rmexample, it has been found desirable to detect andrecord the characteristics or about IOOquantaoIgammarayspersecond and toaccompllm this it is desirable that the recording system lmve a time lagnot in excess of a thousandth of a second and preierably less.

Ithasbeenohservedthattheusuallonization chamber containing iour litersof nitrogen under thirty atmosphereh or pressure absorbs about a hundredquanta ot gamma rays per second under averagercom conditions, nothowever giving any detailed information as to the number oi quanta ortheir individual characteristics. only resists:-ingtotalionintionduetothehmdredquanta. The best Geiger-Hillier countersoi the same dimensions absorb about five quanta per second under averageroom conditions and yield a count or the number or quanta absorbed butnoinformation as to the frequency of the gamma rays constit'll eachquantum.

In an fi isation chamber containing approximately 4 liters of nitrogenat around 450 pounds per square inch pressure, and with the electrodesspaced 1 cmtimeter apart and a voltage of 1,000 volts applied to them,about 100 quanta of gamma rayspersecond willbedetected. 11' therecording circuit is sufiiciently rapid and sensitive an independentrecord of each quantum may be made, which record will indicate not onlytie detection oi each quantum but also the total energy and thus thefrequency of the gamma rays constituting it.

It the current throlmh the ionization chamber is observed by measuringthe voltage produced across a resistor having a resistance of the orderoi ohms, it cannot be measured quickly enough by ordinary means topermit distinctions between individual quanta. To permit the measurementto be made with suillciently great speed. a circuit which adds Iltandmay beused. Buchacircuitis describedinthls inventor's application forUnited States patent, Serial No. 311,217, which matured as Patent Numher2,275,747. Such a circuit. with its output connccted to a last recordinggalvanometer will permit records to be made oi impulses lasting veryshort intervals, ii desired, oi the order 01 ime second or less. a

oi the method A more complete understanding or the details andadvantages oi this invention may be had by a consideration or thefollowing description of the preferred embodiment 01 it. It is to beunderstood, however, that numerous modifications in the construction ofthe apparatus and the details may be made without departing from thespirit of the invention. The invention in its broader aspects includessurface exploration as well as exploration in wells or other openings inthe earth and even includes the inspection oi samples brought to alaboratory. The dimensloriboi the detector. the pressure or kind of gasused, the voltage applied and the resistance placed in series with thedetector are all subject varied and another 01 the factors varied tocompensate ior this variation. All these things, however, are changesthat would be expected 01 one skilledinthlsartandhencethemethodandapparatusassomodiflediswlthinthescopeoithisinvention.

In the drawings:

Figure l is a diagrammatic vice for exploring subsurface strata bylowering a detector into a drill hole in accordance with the principlesor this invention:

Figure 2 is a recording such aswouid be taken by the device of thepresent invention;

Figures 3, 4, 5, 8 and 'i are curves showing graphically variousinformation that may be obtained from the curve oi Figure 2; and

Figure 8 is a schematic illustration oi a modified form of exploringcapsule.

As illustrated in Figure 1 a device constructed according to theprinciples of the present invention may comprise a to be lowered into awell bore H which may be either cased or uncased. Within the casing i.there may be positioned an ionization chamber l2 containing a pair oielectrodes II and Il spaced about I centimeter apart and connectedthroush and insulated from the chamber walls. The chamber may desirablybe filled with nitro- Elegant a pressure or around 450 lbs. per squareOutside oi the ionization chamber but within the casing III a battery IImay be provided for the purpose of supplying a potential between theelectrodes of the ionization chamber. This battery preferably has avoltage of around 1000 volts and has its negative pole connected to theouter electrode and its positive pole grounded to the casing. The innerelectrode i3 is connected through a resistance II to the casing tocomplete the circuit.

Ithasbeenioundthatiitheresistorhasa resistance of around 10" ohms andthe ionization chamber a capacity of 2 l0- farads the time lag in thecircuit can easily be overcome by use of the circuit described in theabove mentioned application 311,217.

Across the resistor II the input of a linear ampliiier "is connected andthe output oithe amplifler I! is connected through a cable II to thesurtaceoitheearth. Thiscableservesalsoto support the detecting unit inthe well bore. The amplifier may be any one of the modern high fidelitylinear amplifiers that have a relatively high amplification rati At thesurface or the earth the cable ll passes over a measuring wheel I I and.is wound onto a cable drum 20 which may be rotated by any suitablemeans, not shown, to raise and lower the illustration 01' a dei orcapsule l0 adapted detecting unit in the hole. The conductors in thecable ll are connected through slip rings II and brushes :2 at the cabledrum 20 so that the currents from these conductors pass to a device 23on the surface. The device 23 consist: of a circuit of the typedescribed in application 311 ,217 and an amplifier. The recording tapeoi the galvanometer is driven either mechanically or by an electricaltransmission system such as the "Selsyn motor arrangement 15 from themeasuring wheel is.

A record oi. the type obtained by use of the above device is illustratedin Figure 2. From the curve 01 Figure 2, there may be obtained thefollowing information:

a. The frequency of occurrence of pulses, whatever their nature. (Fig.3.) This is the same data which would be obtained by a Geiger-Mullercounter.

b. The average value of the curve which records the pulses as measuredfrom its base line (Fig. 4) This curve corresponds with that which wouldbe obtained with an ordinary ionization chamber device, of the priorart.

c. A curve which represents the average rate of occurrence of pulses oimagnitude less than some specified magnitude (Fig. 5) not obtainable bymethods of the prior art.

at. A curve which represents the average rate of occurrence of pulsesgreater than some specified magnitude, (Fig. 6) not obtainable bydevices of the prior art.

e. A curve which represents the average rate of occurence of pulseslying in a specified range or within specified ranges of magnitude, notobtainable by methods of the prior art (Pig. 7)

Curves corresponding to a, b, c, d and e may be produced from the curveoi Fig. 2 by graphic means, or by counting, or by means of automaticdevices, as desired. I! desired, curves of Figures 8 to '1 may beproduced at the time the log is made at the well.

In many instances such information is extremely useful in distinguishingbetween adjacent strata so as to find interiaees not otherwisedeterminable, or in tracing particular strata through an area in whichthe strata being traced lie in closeproximity to other very similarstrata.

The modified type of exploring capsule illustrated in Figure 8 is alsousefulin producing signals havin a magnitude proportional to the energyof quanta absorbed. in this device a casing 30 is provided within whichis mounted an evacuated gas-tight envelope ll internally surfaced with alayer oi fluorescent material such as zinc silicate 32 which in turncarries a thin photoelectric film of a photo-active substance such asmetallic caesium II. An electrode 34 mounted axially within the envelopeII is connected to an amplifier 35 corresponding to the am ll abovedescribed. A connection is also provided between the amplifier 3i andthe photoelectric film SI through a potential source 38 which isconnected in a manner such that a constant potential diflerence existsbetween the film and the electrode. It is to be understood that thelayer of fluorescent material 32 is of suiiicient density and thicknessthat the probability o! photon interaction upon exposure to gammaradiation is considerable although beta rays liberated by the photonswill in general exhaust their range of travel within the substance. Thelayer of photo-active material which preferably is caesium is nearlytransparent and 0! microu scopic thinness as is conventional inphotoelectric cells and similar devices.

The physical constants of the unit are adjusted to cause its capacity tobe of the order 40 10- iarads. In operation photons created byinteraction of gamma radiation with the flucrescent layer act upon thephotoelectric layer resulting in migration of electrons into theevacuated interior of the envelope toward the central electrode thuscausing a signal to be transmitted to the amplifier which, aftersuitable magnification, is transmitted to surface equipment, asdescribed in connection with Figure l of the drawings. It is to be notedthat the processes involved whereby the gamma rays are translated intosignals operate at degrees or efiiciency independent of the energy orthe initial quanta, thus the signal transmitted to the recordingequipment is to a reasonable degree of approximation proportional to theenergy of the initially received quanta.

I claim:

1. A method oi determining the nature of substances that comprisesderiving from individual quanta of gamma rays therefrom an ionizationproportional to the energy in each quantum, separately converting theionization caused by each quantum into an electrical impulse,ampliiying, and recording the magnitude of the current corresponding toeach quantum, separately.

2. A method of geophysical prospecting that comprises detecting, at acontinuously varying depth, individual quantum of gamma rays emittedfrom subsurface formations atiaid continuously varying depth, convertingthe substantially total energy or each individual quantum of gamma raysinto an electrical impulse and recording the magnitude of eachelectrical impulse in correlation with an indication of the depth atwhich said gamma rays were detected separately and distinctly from therecord or the preceding and succeeding electrical current.

3. A device for geophysical prospecting that comprises an ionizationchamber containing a pair or electrodes and a gas under pressure, thesize of said chamber and the pressure of said gas being such as to causethe secondary beta particles produced by the absorption of gamma rays tobe totally absorbed by the gas, means to supply an electrical potentialto the'sald electrodes, the voltage so applied and the spacing oi theelectrodes being such as to cause a substantially complete collection ofthe ions due to a beta particle before there exists an appreciableprobability of the occurrence of another beta particle, a resistance inseries with the said source or potential and electrodes, the saidresistance being of such an order of magnitude that the thermal noisewhich it will impart to an amplifier will be negligibie in comparisonwith the magnitude or the impulses which the amplifier k destined toreceive due to the presence 01 beta particles in the ionization chamber,amplifier and recording means operating from the voltage and the rate ofchange of voltage developed across said resistor, the said ampliiyingand recording means having an appropriate electrical irequencytransmission spectrum suitable to accurately represent the impulsesreceived in the ionization chamber.

4. A device as described in claim 3 further characterized inthat thedetecting unit, source oi potential, resistance and amplifier arecontained in a casing adapted to be lowered into an opening in theearth, the recorder is located on the surface

